Heat-resisting steel for exhaust valves

ABSTRACT

The object of the present invention is to provide a heat-resistant steel for exhaust valves, having relatively small Ni content, high mechanical characteristics (for example, tensile strength, fatigue strength, wear resistance and hardness) at high temperature, and excellent oxidation resistance. The present invention provides a heat-resistant steel for exhaust valves, which includes: 0.45≦C&lt;0.60 mass %, 0.30&lt;N&lt;0.50 mass %, 19.0≦Cr&lt;23.0 mass %, 5.0≦Ni&lt;9.0 mass %, 8.5≦Mn&lt;10.0 mass %, 2.5≦Mo&lt;4.0 mass %, 0.01≦Si&lt;0.50 mass %, and 0.01≦Nb&lt;0.30 mass %, with the balance being Fe and unavoidable impurities, in which the steel satisfies 0.02≦Nb/C&lt;0.70 and satisfies 4.5≦Mo/C&lt;8.9.

TECHNICAL FIELD

The present invention relates to a heat-resistant steel for exhaustvalves.

BACKGROUND OF THE INVENTION

An intake valve for introducing mixed gas of fuel and air into acylinder and an exhaust valve for discharging combustion gas outside thecylinder are used in an engine. Among these, since the exhaust valve isexposed to combustion gas having high temperature, a material havinghigh temperature characteristics (for example, high temperaturehardness, fatigue properties, high temperature strength, wear resistanceand oxidation resistance) is used in the exhaust valve. Ni-basedsuperalloy (for example, NCF751), austenitic heat-resistant steel (forexample, SUH35) and the like are known as a material for exhaust valves.

Ni-based superalloy is a material in which γ′ phase has beenprecipitated by aging treatment, thereby enhancing strength at hightemperature and wear resistance. The Ni-based superalloy is expensive,but has extremely high heat resistance. For this reason, a valve usingthis is mainly used in high output engines that are exposed to atemperature of 800° C. or higher.

On the other hand, the austenitic heat-resistant steel is a material inwhich M₂₃C₆ type carbide has been precipitated, thereby enhancingstrength at high temperature and wear resistance. The austeniticheat-resistant steel is poor in high temperature characteristics ascompared with the Ni-based superalloy, but is inexpensive. For thisreason, a valve using this is mainly used in engines that do not requirehigh heat resistance.

Various proposals have been conventionally made for materials suitablefor such an exhaust valve.

For example, Patent Document 1 discloses a heat-resistant alloy forexhaust valves, containing, by weight %, C: 0.01 to 0.2%, Si: 1% orless, Mn: 1% or less, Ni: 30 to 62%, Cr: 13 to 20%, W: 0.01 to 3.0%, Al:0.7% or more and less than 1.6%, Ti: 1.5 to 3.0%, B: 0.001 to 0.01%, P:0.02% or less, and S: 0.01% or less, with the balance being Fe andunavoidable impurities.

Moreover, Patent Document 2 discloses an Fe—Cr—Ni heat-resistant alloycontaining, by weight %, C: 0.01 to 0.10%, Si: 2% or less, Mn: 2% orless, Cr: 14 to 18%, Nb+Ta: 0.5 to 1.5%, Ti: 2.0 to 3.0%, Al: 0.8 to1.5%, Ni: 30 to 35%, B: 0.001 to 0.01%, Cu: 0.5% or less, P: 0.02% orless, S: 0.01% or less, O: 0.01% or less, and N: 0.01% or less, with thebalance being Fe and unavoidable impurities, and the alloy having agiven component balance.

Furthermore, Patent Document 3 discloses a method for producing anautomobile engine valve, comprising subjecting an Fe-basedheat-resistant steel having a composition of Fe—0.53% C—0.2% Si—9.2%Mn—3.9% Ni—21.5% Cr—0.43% N to solution heat treatment at from 1,100 to1,180° C., forging a valve head part at from 700 to 1,000° C., andsubjecting to aging treatment.

This Patent Document describes that when the Fe-based heat-resistantsteel having a given composition is subjected to solution heattreatment, forging and aging treatment under given conditions, a valveface part can be made to have hardness of HV 400 or more.

By recent sudden rise in raw material cost, production cost of anexhaust valve is greatly influenced by the fluctuation in raw materialcost. In particular, because an Ni-based superalloy has large Nicontent, raw material cost and production cost of an exhaust valve madeof an Ni-based superalloy greatly receive the influence of Ni price. Forthis reason, a material in which an amount of Ni is further reduced andfluctuation width of raw material cost is decreased is desired. However,in an Ni-based superalloy, Ni is a forming element of γ′ phase that is astrengthening phase. Therefore, further reduction in the amount of Nimakes high strengthening utilizing γ′ phase difficult.

On the other hand, a carbide precipitation type austeniticheat-resistant steel is difficult to receive the influence of Ni price,but has a problem that high temperature characteristics are poor ascompared with a γ′ phase precipitation type Ni-based superalloy. Tosolve this problem, a material obtained by highly strengthening SUH35(for example, overseas standard LV21-43 steel (SUH35+1W, 2Nb)) is known.However, the LV21-43 steel still has the problems such that structure isdifficult to control and hot workability is poor.

BACKGROUND ART DOCUMENT Patent Document

Patent Document 1: JP-A-2004-277860

Patent Document 2: JP-A-9-279309

Patent Document 3: JP-A-2001-323323

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

An object to be attained by the present invention is to provide aheat-resistant steel for exhaust valves, having relatively small Nicontent, high mechanical characteristics (for example, tensile strength,fatigue strength, wear resistance and hardness) at high temperature, andexcellent oxidation resistance.

Means for Solving the Problems

To solve the above problem, a heat-resistant steel for exhaust valvesaccording to the present invention having the following constitutions.

(1) The heat-resistant steel for exhaust valves comprises:

0.45≦C<0.60 mass %,

0.30<N<0.50 mass %,

19.0≦Cr<23.0 mass %,

5.0≦Ni<9.0 mass %,

8.5≦Mn<10.0 mass %,

2.5≦Mo<4.0 mass %,

0.01≦Si<0.50 mass %, and

0.01≦Nb<0.30 mass %,

with the balance being Fe and unavoidable impurities.

(2) The heat-resistant steel for exhaust valves satisfies0.02≦Nb/C<0.70.

(3) The heat-resistant steel for exhaust valves satisfies 4.5≦Mo/C<8.9(wherein Nb/C represents a ratio of Nb content (mass %) to C content(mass %), and Mo/C represents a ratio of Mo content (mass %) to Ccontent (mass %)).

Moreover, it is preferred that the heat-resistant steel for exhaustvalves satisfies 5.2≦Mo/C≦8.0.

Also, the heat-resistant steel for exhaust valves may further contain0.0001≦(Al, Mg, Ca)<0.01 mass % (wherein (Al, Mg, Ca) represents thetotal amount of Al, Mg and Ca).

Furthermore, the heat-resistant steel for exhaust valves may furthercontain at least one selected from 0.0001≦B<0.03 mass % and0.0001≦Zr<0.1 mass %.

Advantage of the Invention

In an austenitic heat-resistant steel, when solid solution strengtheningelements such as N and Mo, and carbide forming elements such as Nb andCr are optimized, thereby optimizing MX type carbide amount, M₂₃C₆ typecarbide amount and solid solution strengthening amount, high temperaturecharacteristics (wear resistance and impact resistance) are enhanced anda heat-resistant steel for exhaust valves having excellent hotworkability is obtained.

Particularly, when Mo/C falls within a given range, wear resistance isimproved by solid solution strengthening by solid solution strengtheningelements, and additionally impact characteristics are improved by thereduction in carbide amount. Furthermore, when Nb/C falls within a givenrange, Nb type carbide (NbC) amount and size are optimized, and impactcharacteristics are improved. Furthermore, when the solid solutionstrengthening element is limited to Mo, phase stability is secured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a measurement example of working rangetemperature.

FIG. 2 is a view showing the relationship between Nb/C and an impactvalue.

FIG. 3 is a view showing the relationship between Mo/C and 800° C.hardness.

MODE FOR CARRYING OUT THE INVENTION

One embodiment of the present invention is described in detail below.

[1. Heat-Resistant Steel for Exhaust Valves]

The heat-resistant steel for exhaust valves according to the presentinvention contains the following elements, with the balance being Fe andunavoidable impurities. Kinds of elements added, its component range andthe reason for limitation thereof are as follows.

[1.1. Main Constituent Element]

(1) 0.45≦C<0.60 Mass %

C is an austenite stabilizing element, and suppresses the formation ofsigma phase and Laves phase that are harmful phases. Moreover, Cpreferentially bonds to Nb to form MC type carbide. The MC type carbidesuppresses grain coarsening during solution heat treatment and improvesstrength characteristics. Also, NbC is stable carbide, and the presencethereof in the structure suppresses grain coarsening, resulting in theimprovement in hot workability. Moreover, the MC type carbide acts as ahard phase and improves wear resistance. Furthermore, C bonds to Cr toform M₂₃C₆ type carbide, thereby improving wear resistance and strengthcharacteristics. To obtain such an effect, the C content is required tobe 0.45 mass % or more. The C content is preferably more than 0.45 mass%, and more preferably more than 0.48 mass %.

On the other hand, where the C content becomes excessive, the carbideamount becomes excessive, thereby deteriorating workability and impactcharacteristics. Therefore, the C content is required to be less than0.60 mass %. The C content is more preferably less than 0.57 mass %.

(2) 0.30<N<0.50 Mass %

N is an austenite stabilizing element, and acts as a substitute elementof austenite forming elements such as Ni and Mn. Moreover, N has smallatomic radium, and therefore acts as an interstitial solid solutionstrengthening element and strengthens a matrix. Additionally, N multiplyacts with a substitution type solid solution strengthening element suchas Mo and W, and contributes to the improvement in strength. C and N arestrong austenite forming elements and effectively act as a substituteelement of expensive Ni to reduce costs. Furthermore, N further acts toform MX type carbonitride, in place of C site of MC type carbide. Toobtain such an effect, the N content is required to be more than 0.30mass %. The N content is more preferably more than 0.33 mass %.

On the other hand, in the case where the N content becomes excessive, itis difficult to solubilize N in a matrix. For this reason, the N contentis required to be less than 0.50 mass %. The N content is morepreferably less than 0.47 mass %.

(3) 19.0≦Cr<23.0 Mass %

Cr has an action to form a protective oxide coating film of Cr₂O₃ in ause temperature region of an exhaust valve. For this reason, Cr is anessential element to improve corrosion resistance and oxidationresistance. Moreover, Cr bonds to C to form Cr₂₃C₆ carbide, therebycontributing to the improvement in strength characteristics. To obtainsuch an effect, the Cr content is required to be 19.0 mass % or more.

On the other hand, Cr is a ferrite stabilizing element. Therefore,excessive N content destabilizes austenite. Furthermore, excessiveaddition of Cr promotes formation of sigma phase and Laves phase thatare embrittlement phases, resulting in deterioration in hot workability,strength characteristics and impact characteristics. Therefore, the Crcontent is required to be less than 23.0 mass %.

(4) 5.0≦Ni<9.0 Mass %

Ni is added as an austenite stabilizing element. To stabilize austenite,the Ni content is required to be 5.0 mass % or more.

On the other hand, excessive Ni content leads to high costs. Therefore,the Ni content is required to be less than 9.0 mass %.

(5) 8.5≦Mn<10.0 Mass %

Mn is added as an austenite stabilizing element. Mn not only acts as asubstitute element of expensive Ni, but has the effect of increasingsolubility of N. To obtain such an effect, the Mn content is required tobe 8.5 mass % or more.

On the other hand, excessive Mn content leads to high costs. Therefore,the Mn content is required to be less than 10.0 mass %.

(6) 2.5≦Mo<4.0 Mass %

Mo acts as a solid solution strengthening element of a matrix γ phase,and is an element effective to improve high temperature strength. Toobtain such an effect, the Mo content is required to be 2.5 mass % ormore.

The Mo content is preferably more than 3.0 mass %.

On the other hand, excessive Mo content increases deformationresistance. Furthermore, excessive Mo content promotes the formation ofsigma phase and Laves phase that are embrittlement phases, anddeteriorates hot workability and impact characteristics. For thisreason, the Mo content is required to be less than 4.0 mass %. The Mocontent is preferably less than 3.5 mass %.

Incidentally, as the solid solution strengthening element, there is amethod of W addition other than Mo addition. However, the presentinvention limits to Mo addition. Solid solution strengthening amount bysolid solution strengthening elements such as Mo or W greatly depends onatomic weight. Mo has small atomic weight as compared with W, and atomicnumber per unit mass % is large. Therefore, solid solution strengtheningamount is large. For this reason, in the case of obtaining theequivalent solid solution strengthening amount by W addition,precipitation of Laves phase becomes dominant, and the effect equivalentto Mo is not obtained. For this reason, the present invention limits toMo addition to maximally obtain the effect of solid solutionstrengthening.

(7) 0.01≦Si<0.50 Mass %

Si is an effective element as a deoxidizing agent during melting and toimpart oxidation resistance in high temperature range. Furthermore, Sihas the effect of improving strength as a solid solution strengtheningelement. To obtain such an effect, the Si content is required to be morethan 0.01 mass %. The Si content is preferably 0.03 mass % or more.

On the other hand, excessive Si content leads to deterioration inworkability and deterioration in impact characteristics by the compoundhaving low melting point. For this reason, the Si content is required tobe less than 0.50 mass %. The Si content is preferably less than 0.30mass %.

(8) 0.01≦Nb<0.30 Mass %

Nb bonds to C and N, thereby precipitating MX type carbonitride(including MC type carbide, thereinafter the same). The MX typecarbonitride having an appropriate size and an appropriate amountsuppresses grain coarsening after solution heat treatment, and iseffective to improve high temperature strength characteristics and hotworkability. To obtain such an effect, the Nb content is required to be0.01 mass % or more.

On the other hand, excessive addition of Nb promotes formation offerrite, and forms a large amount of coarse carbonitride. A part of thecoarse carbonitride remains even after the solution heat treatment, andthis leads to deterioration in hot workability and impactcharacteristics. For this reason, the Nb content is required to be lessthan 0.30 mass %. The Nb content is preferably less than 0.25 mass %.

Incidentally, elements for forming the MX type compound include Ti andV, other than Nb, but the present invention limits to Nb. The reason forthis is as follows.

Ti has strong bonding force to C and N and crystallizes a large amountof relatively coarse primary crystal MX carbonitride (primary carbide).The primary carbide of Ti is carbide having very high stability, and theprimary carbide does not dissolve even by solution heat treatment.Therefore, the coarse carbonitride greatly affects deterioration inimpact characteristics. Furthermore, Ti has strong bonding force to O,and therefore forms Ti oxide, thereby remarkably deteriorating oxidationresistance of a material.

Moreover, V is effective to improve strength characteristics. However, Vhas strong bonding force to O, and therefore forms V oxide, therebyremarkably deteriorating oxidation resistance of a material.

For the above reason, the MX type carbonitride forming element islimited to Nb from the balance of strength characteristics and oxidationresistance.

[1.2. Sub-Constituent Element]

The heat-resistant steel for exhaust valves according to the presentinvention may further contain at least any one of the followingelements, in addition to the elements described above.

(1) 0.0001≦(Al, Mg, Ca)<0.01 Mass %

Al, Mg and Ca each can be added as a deoxidizing and desulfurizing agentwhen melting an alloy. Al, Mg and/or Ca contribute to the improvement inhot workability of an alloy. To obtain such an effect, the total contentof Al, Mg and Ca is preferably 0.0001 mass % or more.

On the other hand, excessive content of Al, Mg and/or Ca rather tends todeteriorate workability. For this reason, the total content of Mg and Cais preferably less than 0.01 mass %.

(2) 0.0001≦B<0.03 Mass %

(3) 0.0001≦Zr<0.1 Mass %

Both B and Zr are segregated in crystal grain boundary and strengthenthe boundary. To obtain such an effect, the contents of B and Zr arepreferably 0.0001 mass % or more.

On the other hand, excessive contents of B and Zr lead to thedeterioration in hot workability. For this reason, the B content ispreferably less than 0.03 mass %. Furthermore, the Zr content ispreferably less than 0.1 mass %.

Any one of B and Zr may be added, and both B and Zr may be added.

[1.3. Component Balance]

The heat-resistant steel for exhaust valves according to the presentinvention is characterized in that component elements fall within theabove ranges and additionally, the following conditions are satisfied.

(1) 0.02≦Nb/C<0.70

MX type carbonitride having an appropriate size and appropriate amounthas a role of preventing grain coarsening (improvement in hotworkability) by pinning effect. Furthermore, fine MX carbonitride cansuppress the deterioration in impact characteristics. To obtain such aneffect, a ratio of Nb content (mass %) to C content (mass %) (=Nb/C) isrequired to be 0.02 or more.

On the other hand, in the case where Nb becomes relatively excessive toC, Nb dominantly bonds to C and a large amount of coarse primary crystalMX carbonitride is crystallized. The coarse primary crystal MXcarbonitride does not completely disappear even after solution heattreatment, leading to the deterioration in impact characteristics. Forthis reason, Nb/C is required to be less than 0.70.

(2) 4.5≦Mo/C<8.9

In the case where a ratio of Mo content (mass %) to C content (mass %)(=Mo/C) is too small, the amount of Mo solid-solubilized in a matrix isdecreased, and high temperature strength characteristics represented byhigh temperature hardness is deteriorated. For this reason, the Mo/C isrequired to be 4.5 or more. The Mo/C ratio is preferably 5.2 or more.

On the other hand, Cr site of M₂₃C₆ type carbide is substituted with Moin a certain proportion. However, in the case where the Mo/C ratio istoo large, stability of austenite phase is deteriorated or Laves phaseand a phase that are embrittlement phases are precipitated, leading tothe deterioration in impact characteristics or the deterioration inworkability. For this reason, the Mo/C ratio is required to be less than8.9. The Mo/C ratio is preferably 8.0 or less.

[2. Method for Producing Heat-Resistant Steel for Exhaust Valves]

A method for producing the heat-resistant steel for exhaust valvesaccording to the present invention comprises a melting and casting step,a homogenizing heat treatment step, a forging step, a solution heattreatment step and an aging step.

[2.1. Melting and Casting Step]

The melting and casting step is a step for melting and casting rawmaterials added so as to have a given composition. A method for meltingraw materials and a method for casting molten metal are not particularlylimited, and various methods can be used. Melting conditions can be anyconditions so long as components are homogeneous and casting-capablemolten metal is obtained.

[2.2. Homogenizing Heat Treatment Step]

The homogenizing heat treatment step is a step of subjecting an ingotobtained by the melting and casting step to homogenizing heat treatmentstep. The homogenizing heat treatment is conducted to homogenizecomponents of an ingot.

The homogenizing heat treatment conditions select optimum conditionsdepending on components. In general, the heat treatment temperature isfrom 1,100 to 1,250° C. Moreover, the heat treatment time is from 5 to25 hours.

[2.3. Forging Step]

The forging step is a step of plastic-deforming the ingot after thehomogenizing heat treatment into a given shape. The forging method andforging conditions are not particularly limited, and can be any forgingmethod and forging conditions so long as the desired shape can beefficiently produced.

[2.4. Solution Heat Treatment Step]

The solution heat treatment step is a step of subjecting the materialobtained by the forging step to solution heat treatment. The solutionheat treatment step is conducted to cause coarse primary crystal MXcarbonitride to disappear.

The solution heat treatment conditions select optimum conditionsdepending on components. In general, the residual amount of primarycarbide is decreased and the amount of fine carbide in grainsprecipitated during aging treatment is increased, as the solution heattreatment temperature is increased. Therefore, high solution heattreatment temperature is effective to improve fatigue properties.However, in the case where the solution heat treatment is conducted at atemperature higher than 1,200° C., the precipitation of grain boundaryreaction carbide is accelerated in the subsequent aging treatment,leading to the deterioration in characteristics. For this reason, thesolution heat treatment conditions are preferably 1,000 to 1,200° C.×20minutes or more+water cooling or oil cooling treatment.

[2.5. Aging Step]

The aging step is a step of subjecting the material after the solutionheat treatment to aging treatment. The aging step is conducted toprecipitate M₂₃C₆ type carbide.

The aging treatment conditions select optimum conditions depending oncomponents. Although varying depending on components, the agingtreatment conditions are preferably 700 to 850° C.×2 hours or more+aircooling treatment.

[3. Action of Heat-Resistant Steel for Exhaust Valves]

In an austenitic heat-resistant steel, when solid solution strengtheningelements such as N and Mo, and carbide forming elements such as Nb andCr are optimized, thereby optimizing MX type carbide amount, M₂₃C₆ typecarbide amount and solid solution strengthening amount, high temperaturecharacteristics (wear resistance and impact resistance) are enhanced anda heat-resistant steel for exhaust valves having excellent in hotworkability is obtained.

In particular, in the case where Mo/C falls within a given range, wearresistance is improved by solid solution strengthening due to solidsolution strengthening elements, and impact characteristics are improvedby the reduction in carbide amount. Furthermore, when Nb/C falls withina given range, Nb type carbide (NbC) amount and a size are optimized,and impact characteristics are enhanced. Furthermore, in the case wherethe solid solution strengthening element is limited to Mo, phasestability is secured.

EXAMPLES Examples 1 to 34 and Comparative Examples 1 to 14

[1. Preparation of Sample]

Alloys having compositions shown in Tables 1 and 2 were melted in a highfrequency induction furnace, and 50 kg of ingot was obtained. The molteningot was subjected to homogenizing heat treatment at 1,180° C. for 16hours, and then subjected to forging process to obtain a bar materialhaving a diameter of 18 mm. The forged bar material was subjected tosolution heat treatment (ST) of 1,050° C.×30 minutes−oil cooling.Furthermore, the material after ST was subjected to aging treatment (AG)of 750° C.×4 hours−air cooling.

Incidentally, in Comparative Example 2, “Mo/C” represents “W/C”. Thismay be attributed to the reason that regarding solid-solutionstrengthening, W achieves the effect similar to Mo.

Moreover, in Comparative Examples 4 and 5, “Nb/C” represents “V/C” and“Ti/C”, respectively. This may be attributed to the reason thatregarding the formation of carbonitride, V and Ti achieve the effectsimilar to Nb.

TABLE 1 Composition (mass %) C Si Mn Cr Ni Mo Nb N Other Mo/C Nb/CExample 1 0.53 0.11 9.1 20.8 6.1 3.3 0.12 0.42 6.2 0.23 Example 2 0.460.15 8.9 21.1 5.9 3.2 0.09 0.43 7.0 0.20 Example 3 0.50 0.08 9.2 21.46.2 3.4 0.11 0.40 6.8 0.22 Example 4 0.55 0.12 9.2 20.7 5.8 3.6 0.100.39 6.5 0.18 Example 5 0.58 0.09 9.1 21.1 5.8 3.4 0.11 0.41 5.9 0.19Example 6 0.51 0.13 9.3 19.8 6.0 3.1 0.05 0.32 6.1 0.10 Example 7 0.530.11 8.9 21.7 6.2 3.2 0.16 0.39 6.0 0.30 Example 8 0.52 0.11 9.0 21.46.1 3.4 0.22 0.44 6.5 0.42 Example 9 0.50 0.10 9.1 21.5 6.3 2.9 0.140.48 5.8 0.28 Example 10 0.49 0.02 9.3 20.8 5.8 3.0 0.17 0.42 6.1 0.35Example 11 0.54 0.06 8.8 21.0 6.0 3.3 0.13 0.40 6.1 0.24 Example 12 0.500.28 9.0 20.9 6.0 3.4 0.13 0.45 6.8 0.26 Example 13 0.52 0.38 9.0 21.35.9 2.8 0.08 0.38 5.4 0.15 Example 14 0.50 0.09 8.7 21.5 5.9 3.0 0.130.42 6.0 0.26 Example 15 0.53 0.11 9.8 21.8 5.8 3.3 0.08 0.43 6.2 0.15Example 16 0.51 0.12 8.8 20.1 5.6 3.1 0.13 0.45 6.1 0.25 Example 17 0.490.11 9.1 22.6 6.0 3.3 0.13 0.41 6.7 0.27 Example 18 0.51 0.11 9.2 20.95.5 3.1 0.11 0.45 6.1 0.22 Example 19 0.53 0.17 9.0 21.6 8.7 3.2 0.140.41 6.0 0.26 Example 20 0.56 0.10 8.9 21.4 6.1 2.7 0.13 0.38 4.8 0.23Example 21 0.51 0.10 9.0 20.9 6.0 3.0 0.13 0.42 5.9 0.25 Example 22 0.520.13 9.1 22.2 5.9 3.5 0.12 0.43 6.7 0.23 Example 23 0.53 0.09 8.9 20.96.5 3.9 0.14 0.41 7.4 0.26 Example 24 0.53 0.12 9.0 21.0 5.9 3.2 0.040.40 6.0 0.08 Example 25 0.48 0.11 9.0 21.0 5.6 3.3 0.22 0.42 6.9 0.46

TABLE 2 Composition (mass %) C Si Mn Cr Ni Mo Nb N Other Mo/C Nb/CExample 26 0.46 0.11 8.9 21.5 5.9 3.7 0.28 0.43 8.0 0.61 Example 27 0.530.12 9.1 21.1 6.0 3.4 0.12 0.41 Ca: 0.0015 6.4 0.23 Example 28 0.50 0.128.8 20.9 6.3 3.1 0.14 0.43 Al: 0.0020 6.2 0.28 Example 29 0.52 0.14 9.121.4 6.3 3.3 0.16 0.40 Mg: 0.0021 6.3 0.31 Example 30 0.53 0.15 9.2 21.56.1 3.4 0.17 0.42 Zr: 0.0018 6.4 0.32 Example 31 0.52 0.10 9.0 21.4 6.03.2 0.13 0.39 B: 0.0055 6.2 0.25 Example 32 0.52 0.09 8.8 21.2 6.1 3.20.13 0.41 Ca: 0.0011 6.2 0.25 Al: 0.0016 Example 33 0.51 0.11 9.2 20.95.9 3.3 0.12 0.40 Mg: 0.0023 6.5 0.24 Al: 0.0016 Example 34 0.52 0.108.6 21.0 5.8 3.2 0.16 0.39 Al: 0.0015 6.2 0.31 B: 0.0050 Comparative0.49 0.09 8.9 21.2 3.9 — — 0.40 0.0 0.00 Example 1 Comparative 0.50 0.089.0 21.0 4.0 — 2.03 0.45 W: 1.0 2.0 4.06 Example 2 Comparative 0.65 0.719.2 20.9 6.1 4.1 0.10 0.45 6.3 0.15 Example 3 Comparative 0.64 0.34 9.120.7 6.3 3.8 — 0.51 V: 0.82 5.9 1.28 Example 4 Comparative 0.63 0.39 9.021.2 6.1 3.7 — 0.42 Ti: 0.78 5.9 1.24 Example 5 Comparative 0.63 0.3211.8 21.6 7.3 3.0 1.40 0.32 4.8 1.24 Example 6 Comparative 0.62 0.22 8.720.1 10.2 2.9 0.73 0.31 4.7 1.26 Example 7 Comparative 0.61 0.71 9.221.1 5.9 3.2 0.69 0.40 5.2 1.13 Example 8 Comparative 0.58 0.23 8.7 24.36.0 2.0 0.87 0.40 3.4 1.50 Example 9 Comparative 0.55 0.32 8.8 18.7 6.21.0 2.00 0.42 1.8 3.64 Example 10 Comparative 0.60 0.25 8.9 21.7 6.4 1.02.00 0.26 1.7 3.33 Example 11 Comparative 0.56 0.17 9.0 20.8 6.3 2.31.40 0.40 4.1 2.50 Example 12 Comparative 0.61 0.36 9.1 20.8 6.2 3.90.25 0.40 P: 0.25 6.4 0.41 Example 13 Comparative 0.58 0.35 8.9 22.4 4.74.3 0.81 0.30 Cu: 0.4 7.4 1.40 Example 14[2. Test Method][2.1. High Temperature Hardness]

Hardness at 800° C. of the material after the aging treatment wasmeasured under measurement load of 5 kg using high temperature Vickershardness tester. A material having high temperature hardness of 190 (HV)or more was judged as “⊚ (Excellent)”, a material having hightemperature hardness of 150 (HV) or more and less than 190 (HV) wasjudged as “◯ (Good)”, and a material having high temperature hardnessless than 150 (HV) was judged as “Δ (Fair)”.

[2.2. Charpy Impact Test]

A test piece having 10 mm square, a length of 55 mm and 2 mm U notch(according to JIS Z2202) was cut off from each material after the agingtreatment, and subjected to an impact test of 800° C. Incidentally, thistest was carried out in the test content according to JIS B7722. Amaterial having an impact value of 90 (J/cm²) or more was judged as “⊚(Excellent)”, a material having an impact value of 70 (J/cm²) or moreand less than 90 (J/cm²) was judged as “◯ (Good)”, and a material havingan impact value less than 70 (J/cm²) was judged as “Δ (Fair)”.

[2.3. High Temperature High Speed Tensile Test]

A test piece having a diameter of a parallel part of 4.5 mm was preparedfrom the material having been subjected to forging process, andworkability thereof was evaluated with a high temperature high speedtensile tester. The test conditions were temperature rising time up totest temperature: 100 s, holding time at test temperature: 60 s, andcrosshead speed: 50.8 mm/s. After breaking the test piece, a reductionof area at break was measured. A temperature at which the reduction ofarea at break is 60% or more (working range temperature) was obtained ineach material.

One example of the working range temperature is shown in FIG. 1. Amaterial having a working temperature range of 270° C. or higher wasjudged as “⊚ (Excellent)”, a material having a working temperature rangeof 230° C. or higher and lower than 270° C. was judged as “◯ (Good)”,and a material having a working temperature range lower than 230° C. wasjudged as “Δ (Fair)”.

[2.4. Continuous Oxidation Test]

A test piece having 25 mm×13 mm×2 mm was cut off from the material afterthe aging treatment, and subjected to a continuous oxidation test of850° C.×400 hours. A material having oxidation weight gain of 1.6(mg/cm²) or less judged as “⊚ (Excellent)”, a material having oxidationweight gain of more than 1.6 (mg/cm²) and 2.5 (mg/cm²) or less wasjudged as “◯ (Good)”, and a material having oxidation weight gain morethan 2.5 (mg/cm²) was judged as “Δ (Fair)”.

[3. Result]

[3.1. High Temperature Hardness, Impact Value and Working RangeTemperature]

The high temperature hardness, impact value and working rangetemperature are shown in Tables 3 and 4. The relationship between Nb/Cand the impact value is shown in FIG. 2. Furthermore, the relationshipbetween Mo/C and 800° C. hardness is shown in FIG. 3. The followingfacts are understood from Table 3, Table 4, FIG. 2 and FIG. 3.

(1) Comparative Example 1 having the composition corresponding to SUH35is that the working range temperature is wide, but both the impact valueand the high temperature hardness are low. Moreover, Comparative Example2 having the composition corresponding to LV21-43 steel is that theimpact value and the high temperature hardness are low, and the workingrange temperature is narrow.(2) Comparative Example 3 is that the high temperature hardness is high,but the impact value is low and the working range temperature is narrow.Moreover, in all the Comparative Examples 4 to 12, the high temperaturehardness and the impact value are low, and the working range temperatureis narrow. This may be attributed to the reason that the components andthe component balance are not proper.(3) Comparative Example 13 in which P was added is that the impact valueis particularly low. This may be attributed to the reason thatcoarsening of the precipitated carbide after the aging treatment occursby the addition of P.(4) Comparative Example 14 in which Cu was added is that the workingrange temperature is particularly low. This may be attributed to thereason that a melting point of the material is decreased by the additionof Cu.(5) In all the Examples 1 to 34, the high temperature hardness and theimpact value are high, and the working temperature range is wide.(6) Particularly, in exhaust valves, a sheet material is provided on acontact surface to a valve on the mechanism of an engine in order toseal the inside of a cylinder and holding the state. In adhering betweenthe sheet material and the valve, high stress is applied to an underheadpart of the valve. To suppress premature failure by the stress appliedto the underhead part, the impact value is an important index. Becauseall the Examples 1 to 34 have high impact value, the premature failureis suppressed, and long-life can be achieved.(7) As shown in FIG. 2, when Nb/C is limited to the range of from 0.02to 0.70, high impact value of 90 J/cm² or more is obtained.(8) As shown in FIG. 3, when Mo/C is limited to the range of from 4.5 to8.9, the high temperature hardness of about 190 (HV) or more isobtained. Furthermore, when the Mo/C is limited to the range of from 5.2to 8.0, the high temperature hardness is further improved (about 1 to 5(HV)).

TABLE 3 Working range Impact value Hardness temperature (J/cm²)Evaluation (HV) Evaluation (° C.) Evaluation Example 1 96.0 ⊚ 191 ⊚ 300⊚ Example 2 105.3 ⊚ 197 ⊚ 320 ⊚ Example 3 102.7 ⊚ 195 ⊚ 310 ⊚ Example 496.2 ⊚ 194 ⊚ 300 ⊚ Example 5 90.2 ⊚ 197 ⊚ 280 ⊚ Example 6 97.3 ⊚ 192 ⊚300 ⊚ Example 7 96.4 ⊚ 191 ⊚ 310 ⊚ Example 8 96.7 ⊚ 194 ⊚ 310 ⊚ Example9 97.0 ⊚ 196 ⊚ 300 ⊚ Example 10 99.3 ⊚ 191 ⊚ 310 ⊚ Example 11 94.5 ⊚ 192⊚ 310 ⊚ Example 12 99.3 ⊚ 194 ⊚ 270 ⊚ Example 13 92.1 ⊚ 194 ⊚ 270 ⊚Example 14 95.3 ⊚ 195 ⊚ 310 ⊚ Example 15 95.2 ⊚ 193 ⊚ 300 ⊚ Example 1698.7 ⊚ 193 ⊚ 300 ⊚ Example 17 101.3 ⊚ 195 ⊚ 300 ⊚ Example 18 96.4 ⊚ 193⊚ 300 ⊚ Example 19 95.2 ⊚ 193 ⊚ 290 ⊚ Example 20 93.2 ⊚ 192 ⊚ 310 ⊚Example 21 99.8 ⊚ 193 ⊚ 300 ⊚ Example 22 95.8 ⊚ 196 ⊚ 300 ⊚ Example 2393.2 ⊚ 198 ⊚ 300 ⊚ Example 24 98.8 ⊚ 194 ⊚ 320 ⊚ Example 25 103.1 ⊚ 191⊚ 280 ⊚

TABLE 4 Working range Impact value Hardness temperature (J/cm²)Evaluation (HV) Evaluation (° C.) Evaluation Example 26 101.6 ⊚ 191 ⊚280 ⊚ Example 27 95.8 ⊚ 194 ⊚ 340 ⊚ Example 28 101.3 ⊚ 193 ⊚ 330 ⊚Example 29 96.3 ⊚ 194 ⊚ 330 ⊚ Example 30 97.3 ⊚ 192 ⊚ 330 ⊚ Example 3198.3 ⊚ 193 ⊚ 330 ⊚ Example 32 96.2 ⊚ 193 ⊚ 340 ⊚ Example 33 95.8 ⊚ 193 ⊚330 ⊚ Example 34 96.4 ⊚ 194 ⊚ 340 ⊚ Comparative 77.7 ◯ 140 Δ 340 ⊚Example 1 Comparative 42.3 Δ 160 ◯ 250 ◯ Example 2 Comparative 39.0 Δ200 ⊚ 220 Δ Example 3 Comparative 63.2 Δ 180 ◯ 220 Δ Example 4Comparative 53.1 Δ 184 ◯ 190 Δ Example 5 Comparative 66.6 Δ 182 ◯ 210 ΔExample 6 Comparative 65.2 Δ 180 ◯ 210 Δ Example 7 Comparative 56.8 Δ182 ◯ 190 Δ Example 8 Comparative 70.3 ◯ 178 ◯ 200 Δ Example 9Comparative 37.2 Δ 170 ◯ 190 Δ Example 10 Comparative 40.1 Δ 166 ◯ 190 ΔExample 11 Comparative 45.3 Δ 176 ◯ 210 Δ Example 12 Comparative 28.8 Δ198 ⊚ 100 Δ Example 13 Comparative 54.2 Δ 175 ◯ 180 Δ Example 14[3.2. Continuous Oxidation Test]

A part of the results of the continuous oxidation test is shown in Table5. The following facts are understood from Table 5.

(1) Comparative Examples 4 and 5 in which V and Ti that are MX typecarbonitride forming element similar to Nb and the same result isconsidered to be obtained were added are that oxidation weight gain islarge as compared with the Examples and other Comparative Examples.Those elements have large bonding force to O as compared with Nb, andtherefore, the formation of an oxide easily occurs. As a result, it isconsidered that oxidation resistance was deteriorated. In other words,Ti and V are impossible to be used as a substitute element of Nb.(2) All the Examples 1 to 34 showed good oxidation resistance.

TABLE 5 Oxidation weight gain (mg/cm²) Evaluation Example 1 1.35 ⊚Example 10 1.46 ⊚ Example 13 1.20 ⊚ Example 16 1.51 ⊚ Example 17 1.22 ⊚Example 23 1.33 ⊚ Comparative 3.44 Δ Example 1 Comparative 4.23 ΔExample 4 Comparative 3.85 Δ Example 5 Comparative 2.24 ◯ Example 10

Although the present invention has been described in detail and byreference to the specific embodiments, it is apparent to one skilled inthe art that various modifications or changes can be made withoutdeparting the spirit and scope of the present invention.

Incidentally, this application is based on Japanese Patent ApplicationNo. 2011-182987 filed Aug. 24, 2011 and Japanese Patent Application No.2012-112238 filed May 16, 2012, the disclosures of which areincorporated herein by reference in their entities.

INDUSTRIAL APPLICABILITY

The heat-resistant steel for exhaust valves according to the presentinvention can be used in exhaust valves of various engines.

The invention claimed is:
 1. A heat-resistant steel for exhaust valves,consisting of: 0.45≦C<0.60 mass %, 0.30<N<0.50 mass %, 19.0≦Cr<23.0 mass%, 5.0≦Ni<9.0 mass %, 8.5≦Mn<10.0 mass %, 2.5≦Mo<4.0 mass %,0.01≦Si<0.50 mass %, and 0.01≦Nb<0.30 mass %, with the balance being Feand unavoidable impurities; the heat-resistant steel for exhaust valvessatisfying 0.02≦Nb/C≦0.61 and 4.5≦Mo/C≦8.0.
 2. The heat-resistant steelfor exhaust valves according to claim 1, which satisfies 5.2≦Mo/C≦8.0.3. A heat-resistant steel for exhaust valves, consisting of: 0.45≦C<0.60mass %, 0.30<N<0.50 mass %, 19.0≦Cr<23.0 mass %, 5.0≦Ni<9.0 mass %,8.5≦Mn<10.0 mass %, 2.5≦Mo<4.0 mass %, 0.01≦Si<0.50 mass %, 0.01≦Nb<0.30mass %, and 0.0001≦(Al, Mg, Ca)<0.01 mass % with the balance being Feand unavoidable impurities; the heat-resistant steel for exhaust valvessatisfying 0.02≦Nb/C≦0.61 and 4.5≦Mo/C≦8.0.
 4. A heat-resistant steelfor exhaust valves, consisting of: 0.45≦C<0.60 mass %, 0.30<N<0.50 mass%, 19.0≦Cr<23.0 mass %, 5.0≦Ni<9.0 mass %, 8.5≦Mn<10.0 mass %,2.5≦Mo<4.0 mass %, 0.01≦Si<0.50 mass %, 0.01≦Nb<0.30 mass %, and atleast one selected from 0.0001≦B<0.03 mass % and 0.0001≦Zr<0.1 mass %,with the balance being Fe and unavoidable impurities; the heat-resistantsteel for exhaust valves satisfying 0.02≦Nb/C≦0.61 and 4.51≦Mo/C≦8.0. 5.The heat-resistant steel for exhaust valves according to claim 3, whichsatisfies 5.2≦Mo/C≦8.0.
 6. The heat-resistant steel for exhaust valvesaccording to claim 4, which satisfies 5.2≦Mo/C≦8.0.
 7. A heat-resistantsteel for exhaust valves, consisting of: 0.45≦C<0.60 mass %, 0.30<N<0.50mass %, 19.0≦Cr<23.0 mass %, 5.0≦Ni<9.0 mass %, 8.5≦Mn<10.0 mass %,2.5≦Mo<4.0 mass %, 0.01≦Si<0.50 mass %, 0.01≦Nb<0.30 mass %, and0.0001≦(Al, Mg, Ca)<0.01 mass %, and at least one selected from0.0001≦B<0.03 mass % and 0.0001≦Zr<0.1 mass %, with the balance being Feand unavoidable impurities; the heat-resistant steel for exhaust valvessatisfying 0.02≦Nb/C≦0.61 and 4.5≦Mo/C≦8.0.
 8. The heat-resistant steelaccording to claim 7, which satisfies 5.2≦Mo/C≦8.0.